The grandparent No. and now U.S. Pat. No. 5,006,926 descries a method and apparatus for generating a frequency multiplexed multi-carrier high definition television (HDTV) signal including a digital sound and sync (DSS) signal. The parent No. and now U.S. Pat. No. 5,053,860 incorporates baseband and RF processing necessary to reduce the bandwidth of an HDTV source signal to that of a conventional television signal, for example 6 MHz at RF and describes a format for the signal referred to as "HDS/NA-6" which can be used with an IIDTV source signal having for example, 525 lines, 1:1 progressive scanning or 1050 lines, 2:1 interlaced scanning and can be used to implement a "simulcast" approach for broadcasting HDTV television signals wherein maximum advantage is taken of the existing broadcast spectrum.
The so called "simulcast" approach utilizes one conventional television channel to transmit a the HDTV signal (which cannot be received as transmitted by a conventional television receiver), and a second television channel to transmit a conventional television signal which can provide the same programming as the HDTV signal to conventional, e.g. NTSC, receivers. As used herein, the word "conventional" is defined to mean a standard television channel used in terrestrial television broadcasting, cablecasting and recording, for example a 6 MHz television channel. Since the simulcast approach will initially involve allotting additional channel space to each television station, it will be necessary to provide a system which will allow broadcasters to use the so called "taboo" channels in their area.
When television stations are planned and licensed co-channel interference is taken into consideration. Transmission is precluded on a particular channel in a second city, when the same channel is being used for broadcasting in a nearby city, as determined and regulated by the F.C.C. Although there are other taboo channel considerations, they are generally considered in the industry to be receiver-design taboos, and are not fundamentally insurmountable. However, it is the co-channel interference which causes direct one-on-one interference. Transmission of a "simulcast" HDTV signal on channel A would therefore be considered to be an "interferer" with a conventional transmission also transmitting on channel A, and vice versa, if they are being respectively transmitted in cities which are close enough so that channel A is considered a "taboo" channel in one of them. Such interference is usually described in terms of "D to U" ratios; i.e. Desired signal to Undesired signal ratios.
The parent application, which is specifically incorporated herein by reference, describes a method and system for providing a frequency multiplexed single channel HDTV signal suitable for conventional terrestrial and cable AM broadcasting environments. The signal features a configuration in which carriers may be suppressed and standard sync headroom eliminated. In addition, due to the use of heavy expansion of the signal components derived during the encoding process, performance similar to the signal-to-noise ratio of a conventional broadcast signal can be obtained with a significant reduction in carrier level. The grandparent application describes a method and apparatus for converting a plurality of video and audio components derived from an HDTV source signal into an amplitude modulated RF signal. The video components are derived according to the teachings of the references incorporated herein, which share common inventorship and a common assignee. In particular, the following video components are illustrated and will be used in describing the embodiments presented in the instant application. They are derived in accordance with the methods and apparatus taught in U.S. Pat. No. 4,908,697 which issued on Mar. 13, 1990. These components are:
Y1n, which is a "narrow" bandwidth luminance signal component derived from a first TV line;
Y3w, which is a "wide" bandwidth luminance signal component derived from a third TV line;
CRw, for example U1n, which is a "wide" bandwidth chroma difference signal derived from a first TV line;
CRn, for example V3w, which is a "narrow" bandwidth chroma difference signal derived from a third TV line;
LD2, which is a line difference signal component which can be derived, for example by the formula LD=B-(A+C)/2, where B is a value of a pixel at a particular line and A and C are corresponding pixels vertically above and below pixel B respectively;
LD4, which is a second line difference signal component; and
a DSS packet for transmitting digital sync, clamp period and "CD" quality digital audio.
The components are derived from an HDTV source input having a line sequence of lines 1,2,3,4.
In the parent application, each of the video components are expanded by the factor 27/5. The line difference signals, LD2 and LD4, are modulated in quadrature on a first suppressed sub-carrier and interleaved with both luminance components Y1n and Y3w, much the way chrominance is frequency interleaved in an NTSC signal but with a much smaller interleaving penalty due to the low-energy nature of LD and its spectral composition as vertical detail. The double sideband (DSB) narrowband chrominance signal Crn (U1n) is modulated in quadrature with the narrowband luminance signal Y1n (which contains the imbedded, interleaved quadrature modulated LD components) and filtered as a vestigial sideband (VSB) signal on a second suppressed subcarrier. Similarly, components Y3w and Crw (V3w) are quadrature modulated on a third suppressed subcarrier, with Y3w filtered as a VSB signal. Digital sync and sound signals (DSS) are encoded on a fourth sub-carrier. The four sub-carriers are frequency multiplexed with each other to form the HDS/NA-6 signal.
It has been found that peak power occurs most frequently in the vicinity of the suppressed subcarriers which essentially carry the dense, low-frequency content of a typical image. The packaging of the signal shown in the parent application featured the frequency multiplexed components positioned on the RF signal such that the Yn and Y3w subcarriers faced the inside of the channel, with their upper sideband portions facing away from each other. This orientation poses a problem in terms of interference between the HDS/NA-6 signal configured as described in the parent application and a conventional NTSC television signal present on a taboo co-channel.
The invention herein represents an improvement to the system and signal packaging taught in the parent application and provides a solution to the potential problems associated with using taboo channels.
The instant invention has as one object, to minimize interference between television signals on co-channels. Yet another object of the invention is to maximize the use of available television broadcasting channels by minimizing or eliminating co-channel interference on or between so called taboo channels.